1. Field of the Invention
This invention relates to methods of fabricating rock bit inserts having a varying degree of hardness on selected surfaces of the insert.
More particularly, this invention relates to the fabrication of tungsten carbide inserts having one cutting surface harder than an adjacent cutting surface, the insert being designed to be interference fitted into a gage row of a cone of a roller cone rock bit.
In the drilling industry, maintenance of the gage circumference of a borehole is essential to prevent pinching of subsequent rock bits as they are lowered into the formation for continued drilling. If the gage row of inserts in a roller bit becomes worn, the rock bit begins to drill a borehole that is undersized. Tripping out the worn rock bit results in replacement of that rock bit with a new rock bit having a gage diameter that is larger than the gage of the borehole cut by the previous rock bit. Consequently, as the new rock bit is lowered into the formation it becomes pinched, resulting in either catastrophic failure of the rock bit or drastically reduced rock bit life.
2. Description of the Prior Art
It is well known in the prior art to provide hardened cutting tips for cutting tools such as those which are used in milling machines and the like. For example, U.S. Pat. No. 3,790,353, assigned to the same assignee of the present invention, describes a hardfaced wear pad usable, for example, by brazing the wear pad to a digger tooth to provide a hardened surface for the tooth. The tooth generally is fabricated from steel and the wear pad of tungsten carbide is brazed to the tip of the tooth. The tungsten carbide pad provides a hardened surface to prolong the life of the digger tooth.
A more recent U.S. Pat. No. 4,194,790 discloses a cutting tip insert of a rock cutting tool which comprises two hardened layers. The outside layer is at least several units harder on a hardness scale than the base layer. The layered cutting tip is conventionally brazed to the tip of an insert.
The foregoing prior art patents are disadvantaged in that a multi-step process is required wherein the hardened material has to be brazed or welded to the tips of the cutting instruments.
Yet another disadvantage comes to light in that while the hardened tips are applied metallurgically, the heat generated by most metallurgical methods could attack the integrity of the backup insert or cutting tool to the extent where the tool itself is flawed.
The present invention provides a method to fabricate a rock bit insert from tungsten carbide material with selected cutting surfaces of the insert having tungsten carbide of harder composition than the base insert material. The preferred method of fabrication would form a first layer or pad in a hydraulic ram type press and prior to final sintering, the pre-formed wear pad is inserted in a second insert die cavity. The less hard, somewhat tougher insert material is subsequently compacted against the wear pad by a second hydraulic press. The insert is then sintered, integrally mating the wear pad to the basic material of the insert. It is apparent then that there is no heat brazing or welding of one material to another material. The whole insert is integrally sintered, forming a one piece composite insert having desirable wear characteristics uniquely suited to cutting the gage of a borehole.